Answer the architecture pass: the seam attribute reading names, and the speller a third file copied

src/markdown/parse/directive-attributes.ts

@@ -0,0 +1,31 @@
+import type { AdfAttributes } from '../../adf/document.ts'
+import type { AttributeVocabulary } from '../../adf/attribute-vocabulary.ts'
+import type { DirectiveAttributes } from '../directive-syntax.ts'
+import { attributeValue, spellAttributeValue } from '../directive-syntax.ts'
+import { failure, success, type ConvertErrorPath, type Result } from '../../result.ts'
+
+export type Elsewhere = { key: string; slot: 'argument' | 'content' }
+
+export function readVocabulary(
+  type: string,
+  attributes: DirectiveAttributes,
+  vocabulary: AttributeVocabulary,
+  elsewhere: Elsewhere | undefined,
+  path: ConvertErrorPath,
+): Result<AdfAttributes> {
+  const attrs: AdfAttributes = {}
+  for (const [key, spelled] of attributes) {
+    if (key === elsewhere?.key) {
+      const place = elsewhere.slot === 'argument' ? 'as the directive argument' : 'in the content slot'
+      return failure('unsupported-node-shape', `${type} spells its ${key} attribute ${place}`, path)
+    }
+    const kind = Object.hasOwn(vocabulary, key) ? vocabulary[key] : undefined
+    if (kind === undefined) return failure('unsupported-node-shape', `${type} holds no ${key} attribute`, path)
+    const read = attributeValue(spelled.decoded, kind)
+    if (read === undefined) return failure('unsupported-node-shape', `the ${key} attribute of ${type} is no ${kind}`, path)
+    const spelling = spellAttributeValue(read)
+    if (spelling !== spelled.spelling) return failure('unsupported-node-shape', `${type} spells its ${key} attribute as ${key}=${spelling}`, path)
+    attrs[key] = read.value
+  }
+  return success(attrs)
+}